This invention relates to luminescent materials. More particularly, it is concerned with phosphors of the zinc silicate series.
Manganese activated zinc silicate is a well known green emitting cathodoluminescent phosphor identified commercially as type P-1. When manganese is incorporated into zinc silicate, the resulting material is often off-white or dark in appearance. A white bodied manganese activated zinc silicate phosphor can be produced by employing excess silica over the amount stoichiometrically required, as taught by U.S. Pat. No. 2,245,414 to Roberts, or by incorporating small amounts of magnesium into the zinc orthosilicate lattice as disclosed in U.S. Pat. No. 3,416,019 to Kaduk.
The P-1 phosphors, however, are generally characterized by rapid fluorescence decay and can exhibit the visual effect of undesirable flicker when used in such applications as cathode ray display tubes operated at low referesh rates. To improve the persistance of manganese activated zinc silicate phosphors, small amounts of arsenic are added as taught by U.S. Pat. No. 2,554,999 to Merrill et al. Arsenic-containing manganese activated zinc phosphors are identified commercially as type P-39.
While persistance in type P-39 phosphors generally increases with increasing arsenic content, it does so at the expense of brightness. Often, small changes in arsenic content will produce appreciable changes in phosphor brightness. Thus, in producing these phosphors, an attempt is made to effect a compromise between improved persistance on the one hand and diminished brightness on the other. Rigorous control of the arsenic concentration in the silicate phosphors is often difficult owing to the tendency of arsenic compounds to volatilize from the formulation during the repeated high temperature firings often required for the formation of zinc silicate phosphors.
To facilitate the formation of zinc silicate phosphors, it has been the practice to employ alkali metal or alkaline earth metal salts as fluxes during firing steps as taught by U.S. Pat. No. 2,247,192 to Fonda. The use of such fluxes is not desirable, however, in producing arsenic-containing zinc silicate phosphors since fluxes such as lithium chloride can completely quench the beneficial effect of extended fluorescence decay produced by arsenic in such phosphors [see for example Froelich and Fonda in J. Phys. Chem., 46:878-885 (1942)].
It is believed therefore that an arsenic-containing manganese activated zinc silicate phosphor having improved persistence and brightness would be an advancement in the art.